This invention relates in general to digital television systems and in particular to a black expander for the digital video in such a television system.
In many reproduced televised scencs, the contrast may not be optimum and changing the setting of the receiver "contrast control" setting may not produce any improvement. While the optimum contrast of a particular scene is a matter of subjective judgment, circuits are being constructed to produce scenes of perceived optimum contrast. One prior art television receiver has a circuit for automatically expanding the portion of the video luminance signal that corresponds to grey areas of the picture, to make the picture elements in these areas darker. The resulting improvement in contrast, by expanding the grey areas, is considered to yield a beneficial result. That circuit is an analog implementation and operates by seeking out the blackest portion of the video signal and moving it toward, or fixing it at, the receiver black level. Other video signals between the blackest portion and an arbitrary "breakpoint" are proportionately expanded. Video signals above the breakpoint are not affected.
With the advent of digital television receivers, such analog techniques are not applicable. The present invention is directed to a black expander for a digital television receiver and utilizes moving average filters and comparators to alter the video input/output translation characteristic to enhance or expand signals representative of grey areas to move them toward black level and hence make them appear darker.